1. Field of the Invention
The present invention relates generally to the technical field of fiber optic communication, and, more particularly, to compensating for chromatic dispersion that accumulates as light propagates through a communication system's optical fiber.
2. Description of the Prior Art
Increasing demand for low-cost bandwidth in optical fiber communication systems provides motivation for increasing both bit-rate/transport-distance, and the number of wavelength-division multiplexed (“WDM”) channels which an optical fiber carries. A principal limiting factor in high bit-rate, long-distance optical communication systems is chromatic dispersion which occurs as light propagates through an optical fiber. Chromatic dispersion causes a light wave at one particular wavelength to travel through an optical fiber at a velocity which differs from the propagation velocity of a light wave at a different wavelength. As a consequence of chromatic dispersion, optical pulses, which contain multiple wavelength components, become significantly distorted after traveling through a sufficiently long optical fiber. Distortion of optical pulses degrades and loses information carried by the optical signal.
Chromatic dispersion of optical fibers can be characterized by two (2) parameters:                1. a group velocity dispersion (“GVD”) which is the rate of group velocity change with respect to wavelength; and        2. a dispersion slope which is the rate of dispersion change with respect to wavelength.For a typical optical fiber communication system carrying a broad range of wavelengths of light, such as a WDM system or systems with directly modulated lasers or Fabry-Perot lasers, it is necessary to compensate both for GVD and for dispersion slope across the entire range of wavelengths propagating through the optical fiber.        
Over the years, several different types of optical fibers each of which exhibits different chromatic dispersion characteristics have been used in assembling optical communication systems. The dispersion characteristics exhibited by these different types of optical fibers depend on the length of an optical fiber, the type of optical fiber, as well as how the optical fiber was manufactured, cabling of the optical fiber, and other environmental conditions. Therefore, to compensate for chromatic dispersion exhibited by these various different types of optical fibers it is desirable to have a single type of chromatic dispersion compensating device which provides variable GVD and dispersion slope to thereby simplify inventory control and optical communication network management.
Several solutions have been proposed to mitigate chromatic dispersion in optical fiber communication systems. One technique used in compensating for chromatic dispersion, shown schematically in FIG. 1A, inserts a relatively short length of a special dispersion compensation optical fiber (“DCF”) 31 in series with a conventional transmission optical fiber 30. The DCF 31 has special cross-section index profile and exhibits chromatic dispersion which opposes that of the optical fiber 30. Connected in this way, light, which in propagating through the optical fiber 30 undergoes chromatic dispersion, then propagates through the DCF 31 which cancels the chromatic dispersion due to propagation through the optical fiber 30. However to obtain chromatic dispersion which opposes that of the optical fiber 30, the DCF 31 has much smaller mode field diameter than that of the optical fiber 30, and therefore the DCF 31 is more susceptible to nonlinear effects. In addition, it is difficult to use a DCF 31 operating in its lowest spatial mode for complete cancellation both of GVD and of dispersion slope exhibited by two particular types of optical fibers, i.e. dispersion-shifted optical fibers (“DSF”), and non-zero dispersion shifted optical fibers (“NZDF”).
An alternative inline chromatic dispersion compensation technique, shown schematically in FIG. 1B, inserts a first mode converter 33, which receives light that has propagated through a length of the first optical fiber 30, between the first optical fiber 30 and a high-mode DCF 34. After passing through the high-mode DCF 34, light then passes through a second mode converter 35 and into a second length of the optical fiber 30. Similar to the DCF 31 of FIG. 1A, the high-mode DCF 34 exhibits chromatic dispersion which opposes that of the optical fibers 30, while supporting a single higher order spatial mode than that supported by the DCF 31. The mode field diameter of high-mode DCF 34 for the higher order spatial mode is comparable to that of both optical fibers 30. Thus, the mode converter 33 converts light emitted from the first optical fiber 30 into the higher order spatial mode supported by the high-mode DCF 34, while the mode converter 35 reverses that conversion returning light from the higher order spatial mode emitted from the high-mode DCF 34 to a lower order spatial mode for coupling back into the second optical fiber 30. One problem exhibited by the apparatus illustrated in FIG. 1B is that it is difficult to completely convert light from one spatial mode to another. Another problem is that it is also difficult to keep light traveling in a single higher order spatial mode. For this reason, integrity of a signal being compensated for chromatic dispersion by the apparatus illustrated in FIG. 1B is susceptible to modal dispersion, caused by differing group velocities for light propagating in multiple different spatial modes.
Due to the difficulties in mode matching a DCF to various different types of optical fibers 30 in the field, it is impractical to adjust chromatic dispersion exhibited by DCF's to that required by a particular optical fiber 30. In addition, DCF's also exhibit high insertion loss. This loss of optical signal strength must be made up by optical amplifiers. Thus, compensating for chromatic dispersion using DCF's significantly increases the overall cost of an optical communication system.
A different technique, shown schematically in FIG. 2, uses a chirped fiber Bragg grating 42 to provide chromatic dispersion compensation. Differing wavelength components of a light pulse emitted from the optical fiber 30 enter the chirped grating 42 through a circulator 41 to be reflected back towards the circulator 41 from different sections of the chirped grating 42. A carefully designed chirped grating 42 can therefore compensate for chromatic dispersion accumulated in the optical fiber 30. The amount of chromatic dispersion provided by the chirped grating 42 can be adjusted by changing the stress and/or temperature of the grating fiber. Unfortunately, a Bragg grating reflects only a narrow band of the WDM spectrum. Multiple chirped gratings 42 can be cascaded to extend the spectral width. However, cascading multiple chirped gratings 42 results in an expensive chromatic dispersion compensation device.
Yet another technique, shown schematically in FIG. 3A, employs bulk diffraction gratings 50 for chromatic dispersion compensation. Specifically, light exiting-the transmission optical fiber 30 is first formed into a collimated beam 51. The bulk diffraction grating 50 is then used to generate angular dispersion (rate of diffraction angle change with respect to the wavelength) from the collimated beam 51. A light-returning device 52, which typically consists of a lens 53 followed by a mirror 54 placed at the focal plane of the lens 53, reflects the diffracted light back onto the diffraction grating 50. Reflection of the diffracted light back onto the diffraction grating 50 converts the angular dispersion into chromatic dispersion. A circulator inserted along the path of the collimated beam 51 may be used to separate chromatic dispersion compensated light leaving the diffraction grating 50 from the incoming collimated beam 51. In the apparatus depicted in FIG. 3a, the amount of chromatic dispersion may be adjusted by varying the distance between the diffraction grating 50 and the lens 53, and/or the curvature of the beam-folding mirror 54. However, the bulk diffraction grating 50 produces only a small angular dispersion. Consequently, using the apparatus depicted in FIG. 3A to compensate for the large chromatic dispersion which occurs in optical communication systems requires an apparatus that is impractically large.
An analogous chromatic dispersion compensation technique replaces the diffraction grating 50 with a virtually imaged phased array (“VIPA”) such as that described in U.S. Pat. No. 6,390,633 entitled “Optical Apparatus Which Uses a Virtually Imaged Phased Array to Produce Chromatic Dispersion” which issued May 21, 2002, on an application filed by Masataka Shirasaki and Simon Cao (“the '633 patent”). As illustrated in FIG. 3B, which reproduces FIG. 7 of the '633 patent, the VIPA includes a line-focusing element, such as a cylindrical lens 57, and a specially coated parallel plate 58. A collimated beam 51 enters the VIPA through the line-focusing cylindrical lens 57 at a small angle of incidence, and emerges from the VIPA with large angular dispersion. In combination with the light-returning device 52 illustrated in FIG. 3A, the VIPA can generate sufficient chromatic dispersion to compensate for dispersion occurring in an optical fiber transmission system. Unfortunately, the VIPA distributes the energy of the collimated beam 51 into multiple diffraction orders. Because of each diffraction order exhibits different dispersion characteristics, only one of the orders can be used in compensating for chromatic dispersion. Consequently, the VIPA exhibits high optical loss, and implementing dispersion slope compensation using a VIPA is both cumbersome and expensive. The VIPA also introduces high dispersion ripple, i.e., rapid variation of residue dispersion with respect to wavelength, which renders the VIPA unsuitable for inline chromatic dispersion compensation.
Another technique which may be used in compensating for chromatic dispersion is an all-pass filter. An all-pass filter is a device that exhibits a flat amplitude response and periodic phase response to an incoming optical signal. Since as known to those skilled in the art chromatic dispersion is the second derivative of phase delay, an all-pass filter may therefore be used in compensating for chromatic dispersion. Typical implementations of all-pass filters in compensating for chromatic dispersion are Gires-Tournois interferometers and loop mirrors. An article entitled “Optical All-Pass Filters for Phase Response Design with Applications for Dispersion Compensation” by C. Madsen and G. Lenz published in IEEE Photonic Technology Letters, Vol. 10, No. 7 at p. 944 (1998) discloses how all-pass filters may be used for compensating chromatic dispersion. Problems in using all-pass filters in compensating for chromatic dispersion include their introduction of high dispersion ripple, or an inability to produce sufficient dispersion compensation for practical applications. Consequently, all-pass filters are also unsuitable for inline chromatic dispersion compensation.
Because compensating for chromatic dispersion is so important in high-performance optical fiber communication systems, a simple adjustable dispersion compensator having low dispersion ripple, relatively low insertion loss, and which can compensate for various different types of chromatic dispersion exhibited by the various different types of optical fibers already deployed in fiber optic transmission systems would be highly advantageous for increasing both bit-rate/transport-distance, and the number of WDM channels carried by an optical fiber.